U-bolts are used in many applications, including to clamp together the axle and leaf springs of an automobile. Such bolts have a general U-shape, typically with two end-threaded legs. In a typical U-bolt clamping system or assembly, each leg is inserted through a hole formed in a bracket and a nut is threaded onto the end of each leg, thereby capturing the bracket. Two or more elements, for example, an automobile axle and leaf springs, may be clamped together by positioning the U-bolt and bracket around the elements. With this accomplished, the nuts are then tightened or torqued until the elements are sufficiently clamped together.
For a U-bolt to perform its function properly, the clamping or compression force it applies to the elements must exceed any external forces working to separate the elements. The amount of clamping force exerted by the U-bolt assembly against the elements is dependent upon how much torque is applied during the tightening of the nuts. Not only is it desirable to apply enough clamping force to overcome external forces, it is also necessary to avoid over torquing the nuts beyond the ultimate strength of the U-bolt. Therefore, it is desirable to be able to precalculate the optimum torque to be applied to the nuts of a particular U-bolt assembly in order to choose the proper U-bolt assembly for a particular clamping application.
Previously, the applied torque for U-bolts was calculated by multiplying the tension load needed to be applied to each U-bolt leg (in order to apply the desired clamping load to the elements) by the diameter of one leg and then by a torque coefficient of friction. This relation may be represented by the formula T=W.multidot.d.multidot.K, where T is the torque applied to each nut, W is the tension load for each U-bolt leg, d is the nominal diameter of each leg and K is the coefficient of friction associated with torquing each nut. The torque-tension correlation may be determined by solving for the "K" factor (i.e., K=T/(W.multidot.d)). This formula worked well for determining the appropriate applied torque for standard headed fasteners, such as straight bolts. However, this formula is often inadequate for calculating the appropriate applied torque for U-bolt assemblies.
In general, U-bolts are known for failing at either bend before failing at the threaded ends of their legs. The former method of calculating the optimum torque for a U-bolt has been found unacceptable in accurately calculating the applied torque that will consistently cause the U-bolt to fail. It has been recognized that U-bolts may not sustain a load equivalent to two straight bolts or studs of the same size and grade as the legs of the U-bolt. Thus, in order to determine the actual load carrying capacity of any given U-bolt, the performance of saddle load tests on actual samples of the subject U-bolts has been recommended before the U-bolt assembly is installed. Having to perform such tests before installing the U-bolt assembly is time consuming and costly.
Notwithstanding the prior art, there remains a need for a method of pre-determining the optimum torque for a given U-bolt assembly which is more closely correlated to the actual applied torque, to the necessary clamping force exerted by the U-bolt assembly, and the strength of the U-bolt itself.